1. Field of the Invention
The present invention relates to digital radio communications systems used for mobile communication systems such as car telephones and portable telephones, in particular spread spectrum communication systems based on the CDMA-FDD system that carry out handover between base stations with different carrier frequencies.
2. Description of the Related Art
Mobile communication systems such as car telephones and portable telephones use a cellular system for an effective use of carrier frequencies. In the cellular system, the service area is divided into numerous cells, and in each cell, communication is performed between one base station and a plurality of mobile stations. When a mobile station moves to another cell, it is necessary to switch the communication counterpart from the base station of the old cell to the base station of the new cell. This is called handover between base stations.
FIG. 1 is a processing sequence diagram for handover in a spread spectrum communication system (portable telephone system) based on the conventional CDMA-FDD (Code Divisional Multiple Access-Frequency Divisional Duplex) system. Symbols f1 and C0, etc. in FIG. 1 denote carrier frequency number 1 and spreading code number 0, etc. Since the intention here is to facilitate expansion of base stations, the spreading code timing and frame timing are asynchronous between base stations. Furthermore, continues transmit/receive operations are carried out by taking prevention of interference with hearing aids into account.
The mobile station first calls base station A using carrier frequency number 1 for the down-link, and carrier frequency number 2 for the up-link. Then, the mobile station carries out handover to base station B and enters into communication using carrier frequency number 3 for the down-link, and carrier frequency number 4 for the up-link. The operation during handover between base stations under this condition is explained below.
The mobile station acquires synchronization of a spreading code and frame for down-link synchronization channel (f1, c0) of base station A first. The mobile station then connects down-link communication channel (f1, c1) and up-link communication channel (f2, c2) in that order with base station A to enter into communication with base station A. While communicating with base station A, the mobile station acquires synchronization of a spreading code and frame for down-link synchronization channel of carrier frequency number 3 of base station B (f3, c3) at the same time. After acquiring synchronization with base station B, the mobile station measures the receive level of both base station A and base station B and measures the timing difference between base station A and base station B at the same time.
If the receive level of base station B is greater than the receive level of base station A which is currently communicating, the mobile station issues a handover request to base station A. At this time, it also reports the information of said timing difference to base station A. Base station A reports the handover request received from the mobile station and the timing difference information to base station B via a base station control system. Base station B starts preparations for receiving the mobile station and a down-link communication channel directed to the mobile station is transmitted using carrier frequency number 3.
While maintaining the connection with base station A, the mobile station connects the down-link (f3, c5) and the up-link (f4, c6) of channels for communication with base station B. After a communication channel with base station B is established, the mobile station breaks connection with base station A. That completes handover.
In a portable telephone system based on the conventional CDMA-FDD system, when a mobile station carries out handover to a base station with a carrier frequency different from that of the base station with which it is currently communicating, it must transmit/receive two channels with different carrier frequencies simultaneously. Therefore, the mobile station must be provided with 2 lines of transmit/receive circuits, which entails the problem of increasing both the hardware size of the mobile station and power consumption.
The object of the present invention is to provide a spread spectrum communication system in which a mobile station need not transmit/receive two or more carrier frequencies simultaneously when carrying out handover to a base station with a different frequency.
This object is achieved a spread spectrum communication system in based on the CDMA-FDD system using a plurality of carrier frequencies selectively in which the spreading code timing and frame timing are asynchronous between base stations, including means for carrying out handover to the base station with the same carrier frequency while carrying out handover to a base station with a different carrier frequency and means for switching carrier frequencies within the handover base station after handover between these base stations. When carrying out handover to a base station with a different carrier frequency, this configuration eliminates the necessity of the mobile station transmitting/receiving channels with two or more carrier frequencies simultaneously, allowing the hardware size of the mobile station to be reduced.